A gas-insulated switchgear (GIS) can accommodates high-voltage conductors, such as lead conductors to which a high voltage can be applied. To shield and insulate a high-voltage conductor from other components and from the outside, such an apparatus can include a grounded metal enclosure filled with an insulating gas, for example, a dielectric gas such as SF6.
An insulation spacer can be provided inside a GIS enclosure to hold high-voltage conductors firmly inside the device volume, in a position sufficiently far away from the grounded enclosure to avoid dielectric breakdowns. The insulation spacer can be secured at its outer edge to the enclosure, and has openings for accommodating the high-voltage conductors. A main portion of the spacer can be an insulator disc, with the openings at its center. Some spacers can have a metal armature ring attached to the outer circumference of the insulator disc. The armature ring can have attachment means such as threaded holes, which allow the insulator disc to be attached to the GIS enclosure.
Due to large size and mass, it can be difficult to assemble the armature ring and the insulator disc to form a spacer of sufficiently high stability and precise geometry. It is desirable to have the insulator disc held firmly in the armature ring. To this purpose, some armature rings have two protruding bands at their inner surface (i.e. at the surface directed towards the ring axis and by which the insulator disc is held). The protruding bands run circumferentially around the ring at its inner surface, and the insulator disc can be held between the protruding bands.
Such armatures can be produced by for example, methods such as sand casting. With this method, a part of the casting mold is destroyed when the armature ring is taken out of the mold. Alternatively, the ring inner surface can be milled after casting of the ring. Alternatively, the ring can be cast as several parts, which are assembled after casting. However, these production methods can be costly. Also, it can be difficult to achieve sufficiently low tolerances using these production methods. When the tolerances are too high, however, there can be a risk of stray fields within the GIS, of gas leakage out of the GIS enclosure, so that there is a risk that the lifetime and reliability of the GIS switch may be reduced.